1. Field of the Invention
The present invention relates to a method for cleaning a photo mask to be used for the semiconductor production, in particular, it relates to a method for cleaning a photo mask in a state with a pellicle mounted.
2. Description of the Related Art
According to the photolithography technique used for the production of a semiconductor device or the like, for preventing the transfer of the foreign substances such as the dusts adhered on a photo mask to a wafer, a pellicle is mounted on one side or both sides of the mask using a sticky material or an adhesive. The pellicle comprises a pellicle film made of a fluorine resin film or the like, and a pellicle frame having a predetermined thickness for separating the pellicle film from the mask by a certain distance. The pellicle film is adhered on the upper surface of the pellicle frame so as to form a container-like shape for forming a space with the pellicle and the photo mask (hereafter, it is described as “the pellicle inner space”).
Recently, with the trend of the miniaturization and the high integration of the LSI patterns, according to the photolithography technique to be used for the pattern formation, wavelength of the light source of the exposing device has become shorter from the g line (436 nm) and the i line (365 nm) of a high pressure mercury lamp to the KrF excimer laser (248 nm) and the ArF excimer laser (193 nm). Since such an exposing light source of a short wavelength has high output, the light energy is high, and thereby it causes a phenomenon where foreign substances is grown on the photo mask used for the exposure as the time advances. It is pointed out that the growing of the foreign substances is notable when the exposing light has a shorter wavelength. The foreign substances generated on the photo mask are transferred onto a wafer so as to bring out breakage or short circuit of the semiconductor element circuit.
As to the generation of the growing foreign substances on the photo mask at the time of using a short wavelength exposing light source, as one of the major factors, it is said that a sulfuric acid ion as an acidic substance used for mask washing, remaining on the mask surface after production of the photo mask, and a basic substance such as ammonium present in the mask use environment are reacted by the excimer laser irradiation at the time of the pattern transfer so as to generate ammonium sulfide or the like for providing the foreign substances (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2006-11048, and 2004-53817).
Therefore, even though it is in a preferable quality state without a defect at the inspection after the production of the photo mask, there is a problem in that foreign substances are generated on the photo mask by the repeated excimer laser irradiation with an exposing device, and thereby a good pattern transfer image onto the wafer cannot be obtained.
As to the above-mentioned foreign substances problem, according to a conventional method for washing a photo mask with a pellicle, in the case the growing foreign substances are generated on the mask in the pellicle inner space, since it cannot be washed in a state with the pellicle mounted, the pellicle is separated from the photo mask. And then, for removing the sticky material or the adhesive used for adhering the pellicle on the photo mask, the photo mask is washed with a solution such as hydrogen peroxide sulfate (a solution mixture of sulfuric acid and hydrogen peroxide water) referred to as a piranha solution, or an organic solvent, or a hydrogen peroxide ammonia (a solution mixture of an ammonia water and a hydrogen peroxide water) referred to as a SC-1 solution, and after the washing operation, a new pellicle is mounted again. Moreover, according to the above-mentioned washing method, the pellicle cannot be reused, it is known that a method for washing a photo mask using the pellicle frame capable of dividing it in two for facilitating detachment of the fragile pellicle film. In this method, the pellicle film is detached at the time of washing (see for example JP-A No. 2006-91667).